There's no penalty. You just can't change the lead name yourself online. You have to call MS. If you were just adding, changing or deleting other guests listed on the reservation, you could do that yourself online by using the modify button.
Earlier this year (I think it was April), I was able to change the lead name on a reservation using Chat on the DVC website. Chat isn't always available, but it was easier than long waits on the phone at the time.
I agree with @sethschroeder, I've never seen anything about a date restriction to change name(s) on a DVC reservation.